allow "latest" to be used in release vTag when downloading file (#26748)

Hello,

In the discord I saw
[someone](https://discord.com/channels/322538954119184384/1069795723178160168/1145061200644800514)
complaining that you can't use the "latest" keyword as release tag to
download a specific file:
In his example:

https://www.uberwald.me/gitea/public/fvtt-ecryme/releases/latest/system.json

However the latest keyword works for the release page, so I think it's a
good thing to implement this on the release attachment download url too.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/repo.go

@@ -378,10 +378,6 @@ func RedirectDownload(ctx *context.Context) {
 	curRepo := ctx.Repo.Repository
 	releases, err := repo_model.GetReleasesByRepoIDAndNames(ctx, curRepo.ID, tagNames)
 	if err != nil {
-		if repo_model.IsErrAttachmentNotExist(err) {
-			ctx.Error(http.StatusNotFound)
-			return
-		}
 		ctx.ServerError("RedirectDownload", err)
 		return
 	}
@@ -396,6 +392,23 @@ func RedirectDownload(ctx *context.Context) {
 			ServeAttachment(ctx, att.UUID)
 			return
 		}
+	} else if len(releases) == 0 && vTag == "latest" {
+		// GitHub supports the alias "latest" for the latest release
+		// We only fetch the latest release if the tag is "latest" and no release with the tag "latest" exists
+		release, err := repo_model.GetLatestReleaseByRepoID(ctx.Repo.Repository.ID)
+		if err != nil {
+			ctx.Error(http.StatusNotFound)
+			return
+		}
+		att, err := repo_model.GetAttachmentByReleaseIDFileName(ctx, release.ID, fileName)
+		if err != nil {
+			ctx.Error(http.StatusNotFound)
+			return
+		}
+		if att != nil {
+			ServeAttachment(ctx, att.UUID)
+			return
+		}
 	}
 	ctx.Error(http.StatusNotFound)
 }